After ten years of art education, he went to work for the Romer Grey Studio located in Alta Dena California, a would-be animation shop started by the son of Western author Zane Grey, and financed by Zane Grey's wife. Several cartoons were animated at the Romer Grey Studio, but none were ever released. Most never made it to camera. From there he went to work for Walt Disney. He stayed with Disney's studio for two years before moving to that of Hugh Harman and Rudolf Ising. At that time, he had an accident that gave him a concussion. As a result, he was able to visualise better, thus increasing his production and animation. In 1945, McKimson was promoted to director, replacing Frank Tashlin. He shared this position with Friz Freleng and Chuck Jones from 1949 until 1962, when several other animators had brief directorial stints just before the studio closed over a period from 1963-65. During this period, McKimson created the characters Foghorn Leghorn and Tasmanian Devil, and directed every Hippety Hopper/Sylvester pairing. He also created Speedy Gonzales for the 1953 short Cat-Tails For Two. From 1946 to 1950, Robert McKimson had his very own version of the modern design of Bugs Bunny. Art Davis borrowed it for the rabbit's cameo in The Goofy Gophers and his own Bugs' cartoon. Mckimson also had his own design of Daffy Duck and Porky Pig.
Critics, perhaps unfairly, routinely dismiss McKimson's work — that is, when the critics deem to discuss McKimson's work at all. Much of this critical neglect likely stems from two factors: McKimson's early death, and his extreme shyness. He died well before animation became a respected art form, and when he was alive, he gave few interviews. Also, there have been no theatrically-released anthology films showcasing McKimson's directorial work at Warners (Jones had one, and Freleng had two), which have arguably contributed to his cartoons being less familiar than those of Freleng and Jones.
McKimson is often cited as being a better animator than a director; his shorts are described as having a "squarer" style than his fellow directors, Freleng and Jones. Critics describe his style as somewhat prosaic, literal, and not as innovative, clever or impeccably crafted as the films of Jones or Freleng. In addition, McKimson favored an overstated, hammy style of "acting" for his characters, in contrast to the cool, studied, Method-like underplaying that Jones imbued in his versions of the same characters. In many ways, his cartoons, extremely violent and irreverent, are a continuation of the style of Bob Clampett, who left the studio just after McKimson's promotion to director. His first directorial work, Daffy Doodles (at least his first released directorial work; he cut his directorial teeth on a Seaman Hook wartime cartoon for military audiences in 1944), wherein Daffy draws moustaches on all the pre-drawn (and even some natural) faces in his sight, was released in early April 1946. His cartoons became somewhat more tame after 1950 however, when his story artist, Warren Foster was taken by Freleng in a swap for Tedd Pierce.
But if McKimson's cartoons did not reach the intellectual heights of Jones or enjoy the musical freedom of Freleng, or even have the distinctive cinematic style of Arthur Davis, who took over Clampett's unit in the late 1940s, he is seen by animation scholars as being, individually, the most artistically talented of the Termite Terrace cartoon directors. He may not have had the highs or the lows of Freleng and Jones, but he performed consistently well in the middle. In 1942, McKimson drew a single portrait of Bugs Bunny, leaning against a tree and smiling as he was eating a carrot, that became known as the definitive portrait of the character; this picture has been imitated many times by later artists, including McKimson's peers. McKimson was, for many years, the studio's most prominent animator and character designer; he created the definitive Bugs Bunny model sheet in 1943. Also, in the aforementioned Daffy Doodles, McKimson helped to design a wiser, more society-savvy coal-coloured waterfowl, in marked contrast to the crazy duck of works by Tex Avery and Clampett. His peers acknowledged McKimson's ability to draw images and figures without any construction lines. Even when Warner Bros. acknowledged the influence of UPA and abandoned extreme "realism" in cartoons during the early 1950s, the characters in McKimson's cartoons continued to reflect his craftsmanship, including two of the studio's most popular, Foghorn Leghorn and the Tasmanian Devil.
In 1953, however, the Warner Bros. cartoon studio laid off most of its staff for six months. After the studio reopened, Freleng and Jones were able to quickly re-assemble their respective units, but McKimson lost every member of his previous team, apart from Pierce and background painter Dick Thomas. Some of his post-lay-off cartoons he animated himself. Most of McKimson's weak cartoons were released from 1955 on. These later cartoons have slow timing since McKimson no longer had Rod Scribner or Charles McKimson, his younger brother. At the start of this period, McKimson animated several of his shorts himself. This later period had some merits, including the consistent work of layout artist Robert Gribbroek and occasional brilliance from story man Pierce. However, the McKimson-Pierce spoofs of various television programs, while interesting as period pieces, are badly dated; in general, they have not held up very well against contemporary work by other directors. This is primarily due to the fact that the objects of these parodies are for the most part forgotten, with the noteworthy exception of his three Honey-Mousers cartoons, a take-off on the still-famous Honeymooners creation of Jackie Gleason. Within the studio structure, however, the McKimson unit was valued as the one in which new animators honed their craft, and some of those who excelled (Art Leonardi and Tom Ray, for example) were recruited by Freleng or Jones. Due to McKimson's non-competitive spirit and affability, he simply accepted the arrangement that his unit was the training ground for animators who would then be grabbed up by the studio's two "important" units.
McKimson soldiered on at Warner's cartoon studio as it began to lose people, including Jones, in 1962. Over this time, he directed his share of shorts and worked on the feature The Incredible Mr. Limpet. After the studio closed, he joined DePatie-Freleng Enterprises, co-owned by his old associate Friz Freleng and David H. DePatie, who had been a producer at the Warners studio. At DePatie-Freleng, McKimson directed several The Inspector shorts and worked on some of the Looney Tunes and Merrie Melodies contracted out to DePatie-Freleng by Warner Bros. In 1967, Warners opened its studio again; McKimson went back to Warners in 1968 and stayed until the studio finally shut down for absolute good in 1969. His last Warner Bros. cartoon was Injun Trouble with Cool Cat. Injun Trouble was also the last of the original Looney Tunes or Merrie Melodies cartoon to be produced before the Warner Bros. cartoon studio was closed.
In 1972, he went back to DePatie-Freleng to direct The Pink Panther Show shorts, among their other series.
In his personal life, McKimson was a skilled horseman and polo player, a dedicated bowler, and a master Mason.
McKimson died suddenly in 1977. He suffered a massive heart attack while eating lunch with Friz Freleng and David H. DePatie.