The pair have strong consumer healthcare experience. Mitchell is working for Procter & Gamble, for which Tonic handles pan-European consumer comms, plus GlaxoSmithKline (for global comms on allergy products).
Forni, meanwhile, is working on Welch's Grape Juice and Cadbury Schweppes (on corporate comms).
Clients handled by Mitchell at Biss Lancaster included eye-health firm Bausch & Lomb and Kimberly Clark. Forni promoted brands including Reckitt Benckiser's constipation treatment Senokot.
Mitchell joined Biss Lancaster in 2000 when the agency absorbed Sandpiper PR, which she had just joined. She started her career at Leicestershire-based Claire Bowles Publicity.
Forni formerly worked with Scott Clark and Oliver Parsons – co-founders of 21-strong Tonic – at Fleishman-Hillard.
Biss Lancaster joint MD Jonathan Sanchez said: 'We wish Tonic luck with its new additions.'