Complex sentence: Mark Larson, who joined Digi-Key as its general manager in 1976 and has been its president since 1985, has led the company from its initial focus on the hobbyist market to the expanded market it serves today.
Split into simpler sentences: Mark Larson, who joined Digi-Key in 1976 as its general manager, has been its president since 1985. He has led the company from its initial focus on the hobbyist market to the expanded market it serves today.