The Complete M&A Integration Checklist: Day 1 Through Day 100

Every integration starts with optimism and a slide deck. Most end with missed synergies, departed talent, and confused customers. The difference between the two usually comes down to execution discipline—knowing what to do, when to do it, and who owns it.

This checklist is built from real transactions ranging from $50M tuck-ins to $3B+ enterprise deals. Use it as a starting point. Adapt it to your context. But don’t skip the basics.

Pre-Close: T-30 to Day 0

The work before close determines whether Day 1 is controlled or chaotic.

Integration Management Office (IMO) Setup

Appoint Integration Lead with executive sponsorship and decision authority
Define IMO structure: core team, workstream leads, steering committee
Establish meeting cadence: daily stand-ups, weekly steering, bi-weekly sponsor updates
Create central repository for all integration documentation
Define escalation paths and decision-making protocols
Set up project management tooling and status reporting templates

Day 1 Readiness

Complete employee access provisioning (email, badge, systems)
Prepare Day 1 communications for all stakeholder groups
Brief customer-facing teams on messaging and FAQs
Verify payroll and benefits continuity
Confirm IT infrastructure for acquired employees
Stage welcome materials and logistics for acquired locations
Brief reception and security on Day 1 visitors and protocols

Stakeholder Mapping

Identify top 20 customers requiring personal outreach
Map key supplier relationships and contract implications
List critical employees with retention risk
Identify regulatory stakeholders requiring notification
Document union or works council considerations

Day 1: Establish Control

Day 1 sets the tone. Make it count.

Communications

CEO announcement to all employees (both organizations)
Personal outreach to top 10 customers by identified leaders
Supplier notification with continuity assurance
Press release and media response protocols active
Internal FAQ published and help desk staffed

Operations

All employees can log in and access required systems
Customer orders processing without interruption
Cash management and treasury controls active
Signature authority and approval limits documented
Emergency contact lists distributed

Governance

First IMO daily stand-up completed
Issue log opened and initial items captured
Decision log active
First steering committee meeting scheduled

Days 2-30: Maintain Confidence

The first month is about stability, not transformation.

Weekly Rhythm

IMO daily stand-ups (15 min max)
Workstream leads weekly sync
Steering committee weekly review
Sponsor update (bi-weekly minimum)

Customer Retention

Complete personal outreach to all strategic accounts
Monitor customer service metrics for degradation
Track and address customer inquiries about the deal
Identify any at-risk accounts and develop retention plans
Document customer feedback for operating model decisions

Employee Engagement

Town halls completed at all major locations
Manager toolkit distributed with talking points
Listening sessions scheduled (not just announcements)
HR help desk operational for benefits/policy questions
Early retention discussions with identified critical talent

Financial Controls

Chart of accounts mapping completed
Intercompany transactions identified and tracked
Purchase order and expense approval processes clarified
Month-end close process for combined entity tested
Synergy tracking baseline established

Functional Integration Kickoffs

IT systems assessment and integration roadmap drafted
HR policy harmonization priorities identified
Legal entity rationalization plan developed
Real estate footprint assessment completed
Procurement quick wins identified (duplicate vendors, volume leverage)

Days 31-60: Define the Operating Model

Now the real design work begins.

Organizational Design

Future-state org structure approved for top 2-3 levels
Role clarity documented (who decides what)
RACI charts developed for critical processes
Talent assessment framework defined
Selection process and timeline communicated

Process Integration

Order-to-cash process designed
Procure-to-pay process designed
Record-to-report process designed
Customer service model defined
Escalation paths documented

Technology Decisions

System of record decisions made for core functions
Integration architecture approach selected
Data migration strategy defined
User access and security model designed
Cutover approach and timeline drafted

Synergy Validation

Cost synergy targets validated with bottom-up analysis
Revenue synergy opportunities assessed
One-time costs refined
Synergy capture owners assigned
Tracking mechanisms operational

Days 61-100: Execute and Stabilize

Shift from design to delivery.

Organizational Implementation

Leadership selections announced through Level 3
Workforce planning completed (reductions, redeployments, hiring)
Retention agreements executed for critical talent
Training programs launched for new processes
Change network activated in all major functions

System Transitions

Phase 1 system migrations completed (or clear timeline in place)
User acceptance testing completed for integrated systems
Data quality validated in migrated environments
Business continuity plans tested
Helpdesk prepared for cutover support

Customer Experience

Unified customer-facing processes operational
Account team assignments finalized
Cross-sell/upsell training completed
Customer feedback mechanisms active
NPS or satisfaction baseline established

Synergy Capture

First wave of cost synergies realized
Procurement savings captured
Headcount synergies tracked
Revenue synergy pipeline populated
100-day report prepared for leadership

Ongoing: Sustain and Optimize

Integration doesn’t end at Day 100.

Governance Transition

IMO transitions to business-as-usual ownership
Integration metrics embedded in operating reviews
Lessons learned documented
Remaining workstreams transitioned to functional owners
Deal team retrospective completed

Cultural Integration

Values and behaviors aligned (or differences acknowledged)
Leadership modeling expected behaviors
Recognition programs inclusive of acquired employees
Career paths visible to acquired talent
Engagement survey conducted at 6 months

Performance Tracking

Synergy realization vs. plan reported monthly
Customer retention metrics tracked
Employee retention metrics tracked
Integration cost vs. budget reconciled
Investment thesis validation ongoing

What This Checklist Doesn’t Include

No checklist can replace judgment. This document covers what needs to happen. It doesn’t tell you:

How to handle the politics of leadership selection
When to accelerate vs. slow down based on organizational capacity
Who should make specific decisions for your context
Why certain trade-offs make sense for your specific deal

For that, you need people who’ve done this before—ideally, people who’ve done this before and learned from what went wrong.

The best integration teams use a checklist like this as scaffolding, not scripture. They check the boxes, but they also watch for signals that the plan needs to change.