In 1943, Casey returned to Houston to become Harris County's assistant district attorney. Casey first ran for office in 1948 when he was able to earn a seat in the Texas House of Representatives, representing the 24th district (Houston). However, he chose not to run for reelection to this post, instead spending the next eight years as Harris County Judge (equivalent to a county executive). He was also an administrator at South Texas College.
Casey was elected to the United States House in 1958 in the newly created 22nd district. He defeated the Republican Thomas Everton Kennerly, Sr. (1903-2000) by 43,660 (61.7%) to 23,317 (33%).[2] (Kennerly subsequently went on to be his party's gubernatorial nominee in 1966 against John B. Connally, Jr.)