Mainland was appointed without a pitch for the account, which is thought to be worth up to £100,000 a year.
Quill began working with ABN AMRO Fund Management in 1998 and handled PR around New Star Asset Management Duffield's bid for Artemis in 2001.
Quill continued to work with Artemis after ABN AMRO acquired a 58 per cent stake in the company in 2002.
Mainland founder Neil Mainland worked with Artemis UK equity specialist Mark Tyndall and small-cap expert John Dodd when they were at Finsbury Asset Management.
Tyndall and Dodd co-founded Artemis in 1997 with former Ivory & Sime's fund manager Derek Stuart, who specialises in UK small to mid caps, and Artemis technology and biotech specialist Lindsay Whitelaw.
Two weeks ago, Dodd and Whitelaw closed their £40m target for Artemis's VCT2 venture capital fund.