How do I reduce client concentration before selling my agency?

One client is a huge chunk of revenue and I know a buyer will hate it

Grow the rest of the book faster than the big client and deepen your hold on that account, so its share falls and its risk drops. Buyers heavily discount, or walk from, an agency where one client is a large slice of revenue, because losing it post-deal could sink the business. Diversifying takes time, which is another reason to prepare years ahead.

The Long Answer

Client concentration is one of the sharpest red flags in agency diligence, because it's an existential risk a buyer is inheriting: if one client is 30% of your revenue and they leave a year after the deal, the business the buyer paid for has just lost a third of its income. So they price that risk in hard, through a lower multiple, a chunk of the consideration held back in escrow tied to that client staying, a longer earn-out, or simply walking away.

Reducing it isn't about firing the big client, it's about shrinking their share by growing everything else and by making that relationship stickier and less dependent on you personally. Pour energy into new business and into growing your mid-sized accounts so the denominator rises and the concentration percentage falls naturally.

At the same time, de-risk the big account itself: move the relationship off you and onto a senior team (so it's institutional, not personal), embed yourself deeper into their operations so switching is harder, and get the contract onto a longer, firmer footing if you can. This all takes time, often more than a year to materially shift the numbers, which is precisely why the founders who exit well start preparing two or three years out rather than the quarter they decide to sell. If you're close to a process now, talk to a corporate finance adviser about how to present and structure around the concentration, because how it's handled in the deal matters as much as the raw number.