Complex sentence: He served as Under Secretary of the Treasury from February 3,1961, until April 10,1964, when he returned to private law practice as senior member of the Washington firm of Fowler, Leva, Hawes and Symington.
Split into simpler sentences: He served as Under Secretary of the Treasury from February 3,1961, until April 10,1964. He then he returned to private law practice as senior member of the Washington firm of Fowler, Leva, Hawes and Symington.