Mordechai was apprenticed to Cirna in 876, at the age of 12. Cirna regarded him as her best student ever (an opinion which holds true to this day, though she's had many apprentices since him). He graduated to adept status at age 18, in 882. By 885, he had attained the level of master-adept.
Mordechai remained close to his former master, and he was among the first people she tried to recruit in support of The Plan, in 898. However, in spite of his respect for her, he had mixed feelings about the goals of the Plan, and while he wished her luck, he declined to take an active part in either helping or opposing the Plan. He was even more concerned when, the following year, Durell began sending out Sorreters to work with the gangs which were developing in other villages. He warned Cirna that Durell would likely be rewarded for this by the Planners, but she didn't heed this warning, feeling it would be better to take no action without their prior approval. (In 900, she would come to regret this decision.)
In 902, Amelia visited Sorret, for reasons which remain unknown. While there, she met Mordechai, and the two soon discovered that they shared similar feelings about the Coming of the Order. While the InterGang capo had allowed herself to be used as a pawn in The Order's plans (as they suited her own ambitions), she respected Mordechai's choice to stay out of it. The two soon became friends, and he agreed to leave Sorret with her, and become the First Sorreter of InterGang (to whom the Chief Sorreters of each village's branch of the gang would report). Mordechai is reportedly the only Sorreter she's ever trusted to take to her secret headquarters, wherever they may be. He has provided her with many useful services over the years, including translocating her to various villages as necessary, as well as developing a spell called bittrickle, and providing Amelia with various glamour masks.
In 904, when the surname law was passed, Mordechai chose the name T'Magus. It is known that "magus" is a word sometimes used on Earth for magic users (like Sorreters, but presumably fictitious), though it isn't known what the T stands for in his name. The most common speculation is that it's a contraction of "Top Magus," though other theories have been suggested. Mordechai himself has never commented; or if he has, those he's told haven't revealed the truth.
There are also rumors that Mordechai has had some contact with the Protestant ex-Sorreters who disappeared in 903, though such rumors provide no plausible explanations of when or how such contact might have come about. An unrelated, but much more plausible rumor, is that at some point Mordechai and Amelia's friendship grew into a romantic relationship.