Mary Henry was one of the best abstract painters in the united states, and had been for more then 4 decades. In her precise, modest, and intellectual way, she was as good as Noland, Riley, early Stella, Jack Bush or Ellsworth Kelly. You have never heard of her because:

a) she was a woman, and we still think of women as less good then men, but you have heard of Riley.
b) she was the wrong age, she was born in 1921, she made work just before the post painterly abstraction hit plus she was just a little bit too old to have the kind of revival that Elizabeth Murrary had before she died, or that Mary Heliman is having now. (it means something that Martin or Riley or Henry were left out of the WACK catalog, but Heliman was included)
c) plus she kept making work in seattle, outside of new york or los angeles (though she spent time in the 60s in california), and though noland,louis, and truitt all made work in DC, they had a major champion, and besides DC is a comfortable train ride to new york. (she could paint big though, After Scaleriti, in Tacoma is 20 feet long), Riley had the power of a movement, Martin had her spirtuality, Henry had none of hose things. it was small and unideological
e) she had no friends in the major critics (though she was taught by Nagy), no lovers in the major musuems, she was too pretty, too small, too decorative, and at the end too fucking good.

only real major peice of criticism of her for the stranger in seattle.