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I’m currently working in Equity Capital Markets at a BB and my ultimate goal is to go to law school and secure an in-house counsel legal role. I’ve heard that M&A experience in Big Law is particularly valuable for this path.

My question is whether it’s worth trying to transition into a coverage group at my BB to get M&A experience and build my resume now, or should I just focus on getting into a T14 law school and then trying to land a Big Law position in an M&A group post-graduation?

I think M&A experience as an investment banker would be very helpful in M&A big law, but I’m not sure if it is worth the extra years and decrease in WLB. I’m also unsure how competitive it is to break into an M&A group in Big Law. I know I’m thinking very far ahead, but would love to get any advice or mentorship on the best route to take. Thanks in advance!

Unpacking a few items here:
1. Yes, Biglaw M&A or Cap Markets is the most common prereq for standard in-house counsel at midsize private company/large pubco, if that's your target.
2. Ask no less than 5 lawyers at your BB (or lawyers who you otherwise come into contact with at work) out to coffee and tell them you're interested in going to law school. I'd guess you'll have a high hit rate from cold calling, we like talking about law school and ourselves. The reason to do this is to ensure you want be a lawyer, as you seem to already be on a great career track.
3. If the only reason you'd switch groups is to build your resume for future M&A Biglaw - hard stop, don't do that.

To sum:

I think M&A experience as an investment banker would be very helpful in M&A big law, but I’m not sure if it is worth the extra years and decrease in WLB

Correct, it is not worth the extra years and decrease in WLB.

I’m also unsure how competitive it is to break into an M&A group in Big Law.

Generally one of the less competitive groups at a firm (due to perceived worst WLB and a high number of available slots), but obviously will be competitive to get to a top firm and the M&A group therein.

should I just focus on getting into a T14 law school and then trying to land a Big Law position in an M&A group post-graduation?

Yes, if you have done your due diligence and are confident you want to be a lawyer!