Tomkins, who has worked at Ruder Finn and Weber Shandwick, joins in May from Anglo-Swedish drugs firm AstraZeneca, where she has spent 18 months promoting lung-cancer drug Iressa.
Evans, married to Manning Selvage & Lee director Tim Moss, will leave in May 'to take a break for six months to a year'.
She added: 'Tim and I have always had similar roles and have juggled them successfully for years - but we have been travelling so extensively that something had to give for one of us.'
Tomkins will join an agency that has only just brought in former Roche UK PR manager Rebecca Hunt as its UK healthcare director. Hunt, who last week replaced Jo Fensome - now at Ogilvy PR Worldwide in New York - will report to Tomkins.
AZ withdrew its European marketing application for Iressa in January after the release of long-term data that showed it to be no better than a placebo in prolonging patients' lives.
The news was a 'shock' to AZ, Tomkins conceded.
AZ global director of product PR Margaret Morgan said: 'Lee came to do a specific project, which she has completed.'
She added that Tomkins would not be replaced directly but AZ was 'actively recruiting for the global product PR group both in the UK and Sweden'.
Evans joined PN from GCI in 1997; she became European healthcare practice leader in 2003.
PN has clients that include Novartis and Pfizer/Boehringer Ingelheim, as well as GlaxoSmithKline.