Several folks have asked me, either by email or in comments, to write about this subject: When a partner moves laterally from one law firm to another, will clients stick with the old firm or follow the partner?
I’m afraid the answer is both obvious and indefinite: It depends.
As a client, suppose I have one partner and one associate at a big firm who have handled several cases for me over time. The partner and associate decide to move together to a new firm. Do I follow?

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Of course.
In this situation, the identity of the firm is irrelevant. We probably started working with the partner years ago, and we were happy with her work. She works with only one associate, and we’re happy with the associate’s work. We know the two individuals well, and we know that they personally are doing the work for us.
This is thus a no-brainer: When the lawyers doing our work move on to greener pastures, we follow them. (I have actual experience with this situation. We’ve had a partner and an associate move on, and we followed them. I’m not speaking hypothetically here.)
Change the situation: A firm participates in a beauty contest. Both the lead partner and the team that participate in the beauty contest impress us. We work with the firm for a few months, and the lead partner disappears for a while: She’s in trial for another client, on an extended vacation, in rehab, whatever. But the firm continues to represent us swimmingly even in the partner’s absence.

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The partner then reappears and promptly announces that she’s changing firms. Do we follow?
Of course not. We hardly know her, and she’s proven irrelevant to our representation. We now have a small team at the firm that’s doing nice work for us; we won’t move on for an unknown quantity that happened to have sat at the head of the table during a beauty contest.
Change the situation again: Suppose I have one partner at the helm of a massive case that’s staffed with 15 lawyers from a firm. The partner changes firms. Do I follow?
That’s a much, much trickier situation. Here, we probably hired the partner because of some unique characteristic — perhaps she’s a renowned trial lawyer. But, with a team of 15 lawyers handling the case, the lead partner may be far removed from the details of the matter, and the knowledge essential to running the case may reside with others. If one partner leaves — even the partner at the helm of the case — we may not follow. (I’m not speaking from experience here; that’s just surmise.)
But partners aren’t silly. A person who’s running a big case is unlikely to leave alone. She’s far more likely to grab a couple of other partners working on our case, and maybe a key associate or two, and then move as a group. In that situation, our decision whether to follow is harder: We must figure out where the knowledge (and talent) lies, and act appropriately.
I’m talking here about litigation matters, which are in my power alley. Corporate relationships may be different. That is: Senior corporate partners may have long relationships with a client’s senior executives or board members. But those senior partners may reside at firms that have been handling the client’s corporate work for decades. A lawyer changing firms in that situation may cause the client some angst — should the client follow the relationship, or stay with the institutional knowledge?
That’s beyond both my experience and my knowledge; I won’t hazard a guess.
My sense is that partners are pretty savvy about self-preservation when they change firms. Folks won’t lightly change firms unless they have a pretty good sense that their key clients will follow. Partners ensure that their clients will follow by changing firms in the company of other key members of their teams. (Precisely how partners pull that off consistent with the fiduciary duties they owe their former firms is another matter. I won’t speculate on that subject.)
When a partner changes firms, will clients follow? It depends. But most partners are, I suspect, clever enough to give their key clients little choice (or desire) to stay behind.
Mark Herrmann is the Chief Counsel – Litigation and Global Chief Compliance Officer at Aon, the world’s leading provider of risk management services, insurance and reinsurance brokerage, and human capital and management consulting. He is the author of The Curmudgeon’s Guide to Practicing Law and Inside Straight: Advice About Lawyering, In-House And Out, That Only The Internet Could Provide (affiliate links). You can reach him by email at [email protected].