Due to move to London (from abroad) and start a position as an associate with a US firm towards the beginning of next year and wondering if anyone has any suggestions on nice areas to live in London? Firm is based around Holborn

Islington, Hampstead / St John's Wood, Primrose Hill. Clapham Junction off Northcote Road is popular. Wimbledon Village very nice but not a great commute. These are all quite premium options but for a US firm associate (assuming they pay US scale or thereabouts) they may be worth checking out.
I've been at a US firm for c. 7 years and recently moved to Earlsfield - between CJ and Wimbledon. Works for me re balancing greenery and commute. I didn't like living in Zone 1 but if your priority is sleep & work then places like Clerkenwell / Farringdon are right on your doorstep.