This is a pretty good biography of Perry Mason by Arthur Bloch for the IMDb:
On at least one occasion, Perry has an associate working in his office, but this appears to be an exception to the usual one-man status of his law firm. For a period of perhaps a year, he allowed law student David Gideon, a former client, use of his law library, and Gideon occasionally helped out on Perry's cases. Keeping this office running are his confidential legal secretary Della Street and receptionist Gertie. Although primarily a criminal lawyer, specializing in murder cases, Perry also frequently handles civil cases. On occasion he even makes an exception to his general distaste for divorce cases. His offices are in Suite 904 of the Brent Building, phone: 213-625-1190.
Perry was in the US Navy during World War II. His favorite hobby, when he gets the time to pursue it, is fishing. He and Della often go out to dinner together, and on at least one occasion she tends to him while he's sick. However, other indications of a romantic relationship between them are lacking.
Perry's frequent approach to a murder case is to investigate it thoroughly and discover some person other than his client whom he can prove committed the crime. In this work, he is assisted by Paul Drake, who runs his own private detective agency. Very often, the greatest obstacle that Perry faces is the tendency of his own clients to lie to him. In one case, a client was convicted and sentenced to death, but still would not admit the truth until Perry discovered it by other means.
Perry is widely respected in the legal world, often called upon to give lectures to law students or other groups. Police and prosecutors have a mixed attitude toward him. His occasional skirting of the law, relying on his thorough knowledge of every possible technicality, makes him a constant source of aggravation to the authorities. However, as they are as devoted to the pursuit of justice as he is, they often make common cause with him to ensure that the truth is revealed and the proper person is punished. This is particularly true of Hamilton Burger, who, when one of his own friends is in trouble with the law, knows that Perry is the best choice for defense attorney.
By 1985 in the TV movies, Perry had been a judge, but resigns from the bench in order to defend Della on a murder charge. For the first three years of his return to private practice, he is assisted in his investigations by Paul Drake Jr., who has continued in his late father's footsteps. In 1989, Perry defends law student Ken Malansky, who assumes the role of legman for Perry and his substitutes into 1995. (In 1993-95, Perry was often away and let other talented, experienced attorneys take over his office and cases.) The primary setting of these cases varies between Southern California and Colorado. The relationship between Perry and Della is still close but not clearly defined.
In the 1930's Mason movies, Perry's relations with the police were considerably more adversarial, and hints of his relationship with Della were more suggestive. He apparently has considerable skill as an amateur cook. Paul Drake does not appear, but in some cases, his role as an investigator is taken by the slightly disreputable Spudsy Drake. In The Case of the Velvet Claws, Perry and Della get married, but this is ignored even in the immediately following films.