Franklin Wendell Welker has been the voice of Fred Jones since the debut of Scooby-Doo, Where Are You? in 1969. Frank began providing the voice of Scooby when the series What's New, Scooby-Doo? debuted in 2002.
As IMDb reports, Frank is "Referred to in Hollywood as a voice god."1
The reason for that status is explained by TV Tropes:
"Welker ranks third (only behind Samuel L. Jackson and Harrison Ford) in the "All Time Top 100 Stars at the Box Office," not as a box office draw, but in terms of the total revenue generated by the films in which he has participated, for the sheer number of voices he has done — over 1,200 — ranging from his work on Scooby-Doo to the present day. (In fact, besides The Movie and the 1988-91 series A Pup Named Scooby-Doo where Carl Stevens did the voice, he has been the only voice of Freddy since the beginning.) Go look him up on IMDB. Depending on your monitor resolution and DPI settings, his resume is anywhere from 50-60 pages long. It'd be easier to list the works where he hasn't performed."2
Regarding Frank's career as a voice actor, Broadway World reports, "His first cartoon job was for Hanna Barbara, voicing Freddy Jones in the legendary Scooby Doo series. It is believed that Frank holds the record for voicing the longest running character in the history of animation, Freddy Jones."3
According to IMDb, Frank "Has done every voicing of Fred Jones for all of the Scooby-Doo animated series with the sole exception of A Pup Named Scooby-Doo (1988). Even in parodies and cameos on different television series, he has always done this voice."
On April 29, 2016, Frank Welker was awarded the Lifetime Achievement Award at 43rd Annual Daytime Creative Arts Emmy® Awards.
Additional information about Frank's career is online @ http://emmyonline.com/day_43rd_lifetime_frank_welker
By the way, Frank hasn't limited the voice of Fred Jones to just Fred Jones. In the pilot episode of Tiny Toon Adventures, Frank provides the voice of an unnamed cartoonist who has the same voice as Fred Jones.4 In the series A Pup Named Scooby-Doo, Fred's Uncle Eddie has the same voice as the adult Fred (Episode: "Chickenstein Lives").
1Frank Welker. IMDb. Retrieved from http://www.imdb.com/name/nm0919798/bio?ref_=nm_ov_bio_sm
2Creator/Frank Welker. TV Tropes. Retrieved from http://tvtropes.org/pmwiki/pmwiki.php/Creator/FrankWelker
3EMMY Honors Voice of Scooby-Doo & Curious George Frank Welker. Broadway World. Retrieved from https://www.broadwayworld.com/bwwtv/article/EMMY-Honors-Voice-of-Scooby-Doo-Curious-George-Frank-Welker-20160412
4The Looney Beginning. Season 1 Episode 1. Tiny Toon Adventures. Retrieved from https://www.watchcartoononline.io/tiny-toon-adventures-season-1-episode-1-the-looney-beginning