In February 1874, Grubb was elected into the First Troop Philadelphia City Cavalry (an independent Company of the PA National Guard that had once been George Washington's personal bodyguard during the American Revolution). He was elected as the 16th Captain of the Troop, a position in which he served for 14 years with a break during his foray into politics. This period saw many critical reforms of the National Guard structure that helped ensure an effective mobilization and deployment for World War One. He resigned from the troop in 1896, but later regretted the decision and requested a commission as a colonel in the Expeditionary Force that went to Cuba in 1898.