As the first fuel of the Industrial Revolution, coal was Pennsylvania's most important mineral and Milton Day Kirk was one of many mining engineers from Lehigh who made "black diamonds" their career. Kirk, a native of Curwensville, Pennsylvania, came to Lehigh after studying at Swarthmore College. Apparently, a number of students in the class transferred from other schools as the Class Book of 1906 made special reference to them: "Insofar as their work for the class and the college has entitled them to our esteem and affection, we take a democratic pride in welcoming them at all times as men of 1906." Regarding Kirk, the Class Book noted that he was an expert golfer and added that he "will make a first-class mining engineer or business man," a prediction born out by his career. While at Lehigh, Kirk was also a member of the Phi Kappa Psi fraternity.
After earning his degree, Kirk became a mining engineer for J.A. Weaver and Co., in Staford, Pennsylvania. In 1910, he became chief engineer for the Ebensburg Coal Company in Ebensburg. From there, he went to the Davis Coal and Coke Company in Cumberland, Maryland, to become assistant to the president. By 1923, Kirk was vice president of the Pittsburgh Terminal Railroad and Coal Company, until the 1940s when he became vice president of the Vesta Coal Company, also in Pittsburgh. He died in St. Petersburg, Florida in 1973.
Both Lehigh and Carnegie-Mellon University, where Kirk's wife Ethel attended, received endowments through the generosity trusts established by the Kirk couple.