Amos Leon Thomas Jr. (October 4, 1937 – May 8, 1999), better known by his stage name Leon Thomas, was an American jazz and blues vocalist, born in East St. Louis, Illinois, and known for his bellowing glottal-stop style of free jazz singing in the late 1960s and 1970s. He had one of the most distinctive, powerful voices in modern music.
Leon Thomas was born Amos Thomas, Jr. on October 4, 1937, in East St. Louis, Illinois. He attended Lincoln High School. Already guesting with local choirs and jamming with contemporaries such as the saxophonist Jimmy Forrest and the guitarist Grant Green, Thomas was spotted by a disc-jockey who invited him to come and scat live on his radio show.
After military service Thomas returned in 1964, he resumed his music career, entertaining Presidents Kennedy and Johnson at their inaugural balls. A move back to New York led to a fateful engagement in 1969. "I was playing in Brooklyn with Randy Weston when Archie Shepp and Pharoah Sanders came by," Thomas told Straight No Chaser magazine. "They began to visit regularly and often jammed with us. Pharoah had this song called `Pisces Moon' which he was playing every night as a theme in New York and he asked me if I could put some lyrics to it. I came up with `The Creator Has a Master Plan'. A classic was born."
In 1969, he released his first solo album for Bob Thiele's Flying Dutchman label, “Spirits Known and Unknown.” Thomas became best known for his work with Sanders, particularly the 1969 song "The Creator Has a Master Plan" from Sanders' Karma album.
Through the 1970s, Thomas recorded a series of critically acclaimed records for Flying Dutchman, and performed with Louis Armstrong ion 1970 also the bands of trumpeter Freddy Hubbard and guitarist Carlos Santana, touring as a member of the Santana band in 1973. Thomas was voted best vocalist by the readers of Downbeat magazine from 1970 to 1973.He later appeared on recordings with saxophonist Gary Bartz and singer Jeri Brown. In the mid 1970s, he adopted "Leon" as his middle name. 
According to Ben Ratliff of The New York Times, Thomas had begun his career "as a straight blues-jazz singer" with a "stout tenor voice", but by the mid 1960s, he "had begun to spend time with young jazz musicians who were looking to Africa, the East and meditation for musical material … Thomas developed his ululating singing style, which has been compared to African pygmy and American Indian singing
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techniques and which he later called 'soularphone.' He believed that his ancestors had given him his elastic throat articulation, he said, and henceforth always used it."
Robert Christgau wrote of the significance behind Thomas's vocal abilities in Christgau's Record Guide: Rock Albums of the Seventies (1981) "He has literally expanded the musical possibilities of the human voice. He is as powerful a jazz/blues singer as Joe Williams or Joe Turner, both of whom he occasionally resembles, as inventive a scatter as Ella Fitzgerald. But that's just the beginning, for despite the generation lag, Thomas beats Turner and Williams in their mode even while singing his own, and he turns scatting from a virtuoso trick into an atavistic call from the unconscious."
Robert Christgau wrote of the significance behind Thomas's vocal abilities in Christgau's Record Guide: Rock Albums of the Seventies (1981) "He has literally expanded the musical possibilities of the human voice. He is as powerful a jazz/blues singer as Joe Williams or Joe Turner, both of whom he occasionally resembles, as inventive a scatter as Ella Fitzgerald. But that's just the beginning, for despite the generation lag, Thomas beats Turner and Williams in their mode even while singing his own, and he turns scatting from a virtuoso trick into an atavistic call from the unconscious."
(Edited from Wikipedia & The Independent)