Mark Watson joined the Civil Service in 1989 as an immigration officer based at Gatwick airport. The following year he met his boyfriend, Ander Da Silva, an actor and model, on a field trip up the Amazon. In October 1991, Da Silva visited him on a six-month visa. Watson illegally used his position as an immigration officer to arrange for Da Silva to stay indefinitely in the UK. When this was discovered he was sent to prison for three months and Da Silva was deported. While on bail awaiting trial he wrote a guide, United Kingdom Immigration Law and Rules and Same-Sex Couples, to help gay people in the same situation. It quotes guidelines which stated:
- "Application for leave on the basis of a (homosexual) relationship fall to be considered as a matter of discretion outside the Immigration Rules. Ministers have indicated that such applications are unlikely to be approved."
In 1993 he set up the an Immigration Group within Stonewall Equality Limited; it later became the UK Lesbian & Gay Immigration Group (UKLGIG). After volunteering with Stonewall for a year he joined the payroll, eventually becoming communications director.
In 2002 he was elected as a Labour member of Croydon Council for the Addiscombe Ward, and served for a time as the Cabinet Member for Equalities. He was defeated in 2006, when Labour lost control of the council to the Conservatives, but re-elected in 2010.
In 2003 he climbed Kilimanjaro to raise money for HIV / AIDS projects in Africa.
In 2011 he founded the Gaily Mail website.
In 2016 he was the main organiser of the first Croydon PrideFest.