Simon first appears in June 1390, when he and his father, Sir John Leek, were given joint custody of land in Derbyshire held by knight service of the latter’s ward, Alice Foljambe. The two men often acted together as trustees, mainpernors and witnesses, most notably for Sir Thomas Chaworth, Sir John Etton, John Markham j.c.p., their kinsman, Sir Thomas Rempston I, and their neighbour, William Sibthorpe, who, in the autumn of 1394, also made Simon one of his executors. Meanwhile, in May 1392, the latter indented to serve in the company of Thomas of Woodstock, duke of Gloucester, the newly appointed lieutenant of Ireland. Notwithstanding the revocation of Gloucester’s commission, Leek received £8 11s.8d. as his fee from Sir John Clifton, under whose banner he had intended to set sail.
The Leeks were a close-knit family, and Simon was constantly involved in the affairs of his various kinsmen. In January 1405, for example, he went surety for John Leek in a dispute over the custody of land belonging to a royal ward, besides appearing frequently as a trustee for his relatives and sometimes as the holder of a reversionary interest in their estates.
Simon was busy during the early 1420s as a trustee of his father’s old friends, Sir John Zouche and Sir Thomas Chaworth, the latter of whom he helped to return to Parliament for the sixth time at the Nottinghamshire elections of 1423. He probably died towards the end of the decade, leaving four daughters and coheirs who also laid claim, through their paternal grandmother, to the estates of John Vaux, and were thus much sought after as marriage partners. The eldest became the second wife of Sir Giles Daubeney (d.1456), while her sisters married, respectively, Sir John Markham, c.j.KB (whose inheritance Simon Leek had long held in trust), Hugh, the son and heir of Sir Thomas Hercy of Grove, and the Lincolnshire landowner, Richard Willoughby.
