Magister works with GPs, the pharmaceutical industry and the NHS to provide healthcare benefits for patients.
Fleetlease will provide contract hire, maintenance, 365-day cover and accident management to the firm, with vehicles on a three-year/ 75,000-mile replacement cycle.
Magister's operations director Andy Perrie said: 'When I joined the company in July 2000, there were seven members of staff and we leased our first seven cars. Now there are 128 staff and the fleet has had to grow rapidly to match this.'
The firm's fleet is expected to increase further to more than 150 vehicles in the next year.
In the past, contract hire firms supplying cars have included HSBC Vehicle Finance and Powermarque.