Mentors - How Important?

From my experience as a summer analyst, you only meet with your mentor 2-3 times the whole summer. Don't place too much importance on it. Do choose groups based on interest as well as the people in there. In short I guess do base your decision on people, but not a particular person, especially if your interactions are limited. Maybe other people had different experiences with mentors but that's just my take. Also sometimes they assign you a mentor from a different group.

Depends on the mentor's role. If the mentor is the person you spend your day to day with, you should definitely make sure it is someone you like and are comfortable with. Otherwise, it probably shouldn't weigh in enough to influence your group preferences.

Generally for summer internships the mentor is not super important... I might meet with my mentees (is that a word? lack of sleep, sorry) once a month or so in the summer, but it's not a critical factor. Just another way to help them get used to banking/the culture etc.

I would agree that the group itself and people in it should weigh much more heavily on your decision-making process.

If another group seems more interesting, go with that one.

Also keep in mind that during the summer, your group selection is not as important as it is for FT jobs where a bad group could screw you for 2 years.

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Mergers & Inquisitions