Complex sentence: He graduated from Los Angeles' Fremont High School in 1936, and by then, had already begun his professional career with the 1934 and 1935 Hollywood Stars of the Pacific Coast League (PCL).
Split into simpler sentences: He graduated from Los Angeles' Fremont High School in 1936. He had already begun his professional career with the 1934 and 1935 Hollywood Stars of the Pacific Coast League (PCL).