The migration call I get most often goes something like this: "We've been on Mailchimp for three years, we have about 8,000 contacts, and we want to move to HubSpot. How hard can it be?"
My answer is always the same: it depends on what you're moving. The contacts are easy — a CSV export, a CSV import, you're done. But if you're moving anything that resembles marketing automation, behavioral data, or complex segmentation, you're not doing a migration. You're doing a rebuild with a data backstop.
Here's the honest breakdown of what transfers, what partially transfers, and what you should plan to rebuild from scratch.
Why Mailchimp Outgrows Itself
Mailchimp is excellent at one thing: sending email newsletters to a list. It was built for that, and it still does that well. The moment your marketing needs go beyond broadcast emails — lead scoring, behavioral triggers, multi-touch sequences tied to CRM data, deal-stage-based follow-ups — Mailchimp starts showing cracks.
The specific failure modes I see most often:
- Automation logic lives in people's heads. When your "welcome sequence" is a series of manually-timed sends rather than a triggered workflow, it only runs when someone remembers to enroll the contact. Half your new subscribers miss it.
- List fatigue is structural. Mailchimp's approach to re-engagement is a one-click "we miss you" email. There's no automated win-back logic, no score-based suppression, no lifecycle-stage-aware triage. Bad addresses and dead subscribers quietly poison your deliverability.
- No real-time behavioral scoring. You can build a simple engagement score in Mailchimp using custom fields updated by automation, but it requires manual maintenance and falls apart the moment your sales team starts logging activity outside the platform.
- Your CRM, your analytics, and your email tool are all siloed. Every piece of data about a contact lives in a different tool — Mailchimp for email, a spreadsheet for lead status, Google Analytics for web behavior. They're never fully in sync, so nobody has a complete picture of any individual prospect.
The teams that migrate successfully are usually the ones who have already hit one or more of these walls. They're not migrating to HubSpot — they're migrating away from the operational debt Mailchimp built up over years of manual workarounds.
What Transfers (and What Doesn't)
What transfers cleanly
Contacts and basic field data. Name, email, phone, company — the standard Mailchimp merge fields export to CSV and import to HubSpot without friction. Most teams do this in a day.
Tags and custom fields. Mailchimp's tagging system maps reasonably well to HubSpot's lists and static lists. Tags become list memberships or contact properties depending on how you want to use them. Custom fields (anything you've built in Mailchimp beyond the standard merge fields) export and import, though some field type conversions (date fields, dropdowns) require manual mapping.
Audience segments. If you've built smart segments in Mailchimp based on tag combinations or merge field values, you can recreate them in HubSpot using static lists and, for more complex logic, HubSpot's list segmentation with filters. It's manual recreation, not a push-button transfer — but the underlying data is there.
Email templates and campaign content. Your existing HTML email templates can be imported into HubSpot's design manager. Mailchimp's drag-and-drop templates translate reasonably well; heavily coded custom templates may need some cleanup. Campaign content — the actual copy you've sent — doesn't import, but you have it if you need to reconstruct anything.
What doesn't transfer
Mailchimp automation workflows. This is the most underestimated gap. Every automation you've built in Mailchimp — the welcome sequence, the birthday email, the post-purchase follow-up, the re-engagement loop — does not exist in HubSpot after migration. You start from zero. The logic, the timing, the conditional branches, the enrollment triggers: all of it is gone unless you've documented it beforehand. This is why a pre-migration audit of your Mailchimp automations is the most valuable thing you can do before a project starts.
Subject line testing history. Mailchimp's A/B testing data stays in Mailchimp. Your winning subject lines, open rate patterns, and send-time optimization data don't transfer. HubSpot has its own A/B testing functionality, but you're starting fresh with a new platform's algorithm and user base.
Subscriber behavior logs and engagement history. This is the one that hits hardest. Mailchimp tracks email opens, click tracks, and unsubscribes per contact — but that history doesn't export in a usable form. You get a date-stamp of last activity, not the full timeline. In HubSpot, you're essentially starting with a clean engagement history for every contact. Your segments will be based on what you know about contacts from Mailchimp data, not what HubSpot tracked automatically.
Embedded form data. If you used Mailchimp's embedded forms to capture leads, the submission data lives in Mailchimp. HubSpot doesn't import form submissions — you need to export them from Mailchimp and import them as contact properties or notes in HubSpot. It's manual and time-consuming for anything beyond a few hundred records.
Custom integrations via API. Anything you've built on top of Mailchimp's API — a custom signup form, a lead sync with a different CRM, a data enrichment pipeline — doesn't transfer. Each integration needs to be rebuilt against HubSpot's API or, preferably, replaced with HubSpot's native equivalent where one exists.
Bottom line: Your contacts transfer. Your tags and segments transfer. Your templates transfer. Everything that represents logic — your automations, your behavioral triggers, your conditional sequences — needs to be rebuilt.
The Migration Timeline
A realistic, properly-scoped Mailchimp to HubSpot migration takes 10–12 weeks from kickoff to full go-live. Here's how that breaks down across five phases.
Audit & Mapping
What happens: Every Mailchimp automation is documented — trigger, logic branches, timing, exit conditions. Contact fields and tags are mapped to their HubSpot equivalents. List segmentation logic is translated into HubSpot list definitions. The goal is a complete blueprint of what currently exists before touching anything in HubSpot.
Who does it: A migration specialist reviews Mailchimp and produces the map. The client reviews and approves. This phase determines the scope of everything after it.
Data Migration
What happens: Contacts export from Mailchimp in batches (all fields, all tags), cleans for duplicates and bounced addresses, and imports into HubSpot via native import or API. Custom fields are mapped. Tags become list memberships or contact properties. Suppressed and bounced addresses are added to a HubSpot suppression list.
Who does it: Migration specialist runs the technical import. Client validates a sample of records for accuracy — field mapping, name formatting, data completeness.
Automation Rebuild
What happens: Each Mailchimp automation is rebuilt in HubSpot using workflows, sequences, and (if needed) HubSpot's Workflow API for more complex logic. This is the longest phase and the one where most teams underestimate effort. A welcome sequence that took 20 minutes to build in Mailchimp can take 8–10 hours to properly rebuild in HubSpot with proper enrollment triggers, conditional branches, and follow-up tasks.
What gets rebuilt first:
- Welcome sequence and new subscriber onboarding
- Post-purchase or post-form follow-up sequences
- Re-engagement loops for cold contacts
- Birthday or anniversary automations
Who does it: Automation specialist builds. Client reviews each workflow in HubSpot's sandbox environment before it goes live.
QA & Training
What happens: End-to-end testing of every workflow: trigger fires, logic runs correctly, emails send with correct content, contact properties update as expected. A sample of contacts from the migrated data is checked against the original Mailchimp records for field accuracy. The team learns HubSpot's workflow builder — not just what was built, but why it was built that way.
Who does it: Migration specialist runs QA. Client team participates in training sessions to own the workflows going forward.
Go-Live & Mailchimp Sunset
What happens: New HubSpot workflows go live for all migrated contacts. DNS is updated to point forms and integrations to HubSpot. Mailchimp is placed in read-only mode for 30 days (catches any remaining automation triggers or API calls) before full sunset. Post-launch monitoring for 2 weeks — email deliverability, workflow errors, contact property accuracy.
Who does it: Migration specialist coordinates the switchover. Client monitors deliverability and flags issues.
The Audit First Principle
The difference between a migration that ships on time with everything accounted for and one that drags into month six comes down to one thing: auditing Mailchimp before touching HubSpot.
I've seen teams budget $15,000 for a migration and spend $40,000 because they discovered mid-build that they had 23 active automations nobody had documented, a list of 2,400 bounced addresses that weren't in any suppression list, and a custom API integration that connected their signup form to three different tools.
A pre-migration audit takes a week. It costs roughly 10% of a typical migration budget. It tells you exactly what you're moving, what you're rebuilding, and how long each phase will take. Without it, you're estimating blindly and paying for the gap between the estimate and reality.
If you're not sure what your migration looks like — what data you have, what automations you're running, what the scope actually is — the fastest way to find out is a free migration audit.