Grimes will meet Corey Weworski, who ousted Thuhashini Selvaratnam 1-up, in Wednesday's 18-hole final.
Grimes, who defeated Lisa DeSimone to earn a spot in the semifinals, took a quick 1-up lead on Tuesday after Hartwiger bogeyed the opening hole. Grimes then birdied the par-4 third to go 2-up.
Hartwiger battled back with a birdie at the par-3 fourth, but Grimes responded with a birdie at the very next hole to regain a 2-up lead.
Hartwiger, who won this event in 2001, fell 3-down after she bogeyed the par-4 10th. Grimes then sealed the deal after Hartwiger found trouble again with a bogey at the 15th.
Weworski, who knocked off Patricia Cornett to reach the semifinals, was trailing after the first nine holes and fell 2-down after a bogey at the 11th.
Selvaratnam struggled at the following hole, however, but got back to 2-up after Weworski bogeyed the 13th. Weworski climbed back and won the 14th and the 15th to square the match.
Weworski then parred the 18th to secure a place in the final against Grimes.
The Women's Mid-Amateur is one of 13 national championships conducted annually by the United States Golf Association, 10 of which are strictly for amateurs.
Corey Weworski def. Thuhashini Selvaratnam 1 up
Virginia Grimes def. Kathy Hartwiger 4 and 3