On September 14, 1957, in the second game of a double-header against the Pittsburgh Pirates, Hillman pitched to Cub catcher Jim Fanning, playing on their shared 30th birthdays.

[3] In 1959 with the Cubs, he posted career-numbers in appearances (39), wins (8), starts (24), complete games (4), strikeouts (88), and innings pitched (191).

On May 6, 1959, at Forbes Field, he threw his only big-league shutout against Harvey Haddix and the Pittsburgh Pirates, 3–0, giving up two hits, walking one and striking out two.

Along with his Cubs teammate Jim Marshall, Hillman was part of the first inter-league trade without waivers in MLB history, when he went to the Boston Red Sox in return for Dick Gernert.

The November 21, 1959, transaction was the first during a three-week period of unrestricted trading permitted by a change in both leagues' rules.