Haggis began to work as a writer for television programs, including The Love Boat, One Day at a Time, Diff'rent Strokes, and The Facts of Life. With The Facts of Life, Haggis also gained his first credit as producer. During the 1980s and 1990s, Haggis wrote for television series including thirtysomething, The Tracey Ullman Show, FM, Due South, L.A. Law, and EZ Streets. He helped to create the television series Walker, Texas Ranger; Family Law; and Due South. Haggis served as executive producer of the series Michael Hayes and Family Law.

He gained recognition in the film industry for his work on the 2004 film Million Dollar Baby, which Allmovie described as a "serious milestone" for the writer/producer, and as "his first high-profile foray into feature film". Haggis had read two stories written by Jerry Boyd, a boxing trainer who wrote under the name of F.X. Toole.

Haggis later acquired the rights to the stories, and developed them into the screenplay for Million Dollar Baby. Clint Eastwood portrayed the lead character in the film. Eastwood also directed the film, and used the screenplay written by Haggis. Million Dollar Baby received four Academy Awards including the Academy Award for Best Picture.

After Million Dollar Baby, Haggis worked on the 2004 film Crash. Haggis came up with the story for the film on his own, and then wrote and directed the film, which allowed him greater control over his work. Crash was his first experience as director of a major feature film. Highly positive upon release, critical reception of Crash has since polarized, although Roger Ebert called it the best film of 2005.

Crash received Academy Award nominations for Best Picture and Best Director, in addition to four other Academy Award nominations. Haggis received two Academy Awards for the film: Best Picture (as its producer), and Best Writing for his work on the screenplay. With Million Dollar Baby and then Crash, Haggis became the first individual to have written Best Picture Oscar-winners in two consecutive years.

Haggis said that he wrote Crash to "bust liberals", arguing that his fellow liberals were not honest with themselves about the nature of race and racism because they believed that most racial problems had already been resolved in American society.

哈吉斯开始担任电视节目的编剧，包括《爱之船》、《一天一天》、《Diff'rent Strokes》和《生活的事实》。凭借《生活的事实》，哈吉斯也获得了他作为制片人的第一个荣誉。在 1980 年代和 1990 年代，哈吉斯为电视连续剧撰稿，包括《三十多岁》、《特蕾西·乌尔曼秀》、《FM》、《正南》、《洛杉矶法律》和《EZ Streets》。他帮助创作了电视连续剧《沃克，德克萨斯游骑兵》;家庭法;和正南。哈吉斯曾担任《迈克尔·海耶斯》和《家庭法》系列的执行制片人。

他因在 2004 年电影《百万美元宝贝》中的工作而获得电影界的认可，Allmovie 将其描述为编剧/制片人的“重要里程碑”，也是“他首次高调涉足故事片”。哈吉斯读过杰里·博伊德（Jerry Boyd）写的两个故事，杰里·博伊德（Jerry Boyd）是一位拳击教练，以F.X.图尔（F.X. Toole）的名义写作。

哈吉斯后来获得了这些故事的版权，并将它们发展成《百万美元宝贝》的剧本。克林特·伊斯特伍德在电影中饰演主角。伊斯特伍德还执导了这部电影，并使用了哈吉斯写的剧本。《百万美元宝贝》获得了包括奥斯卡最佳影片奖在内的四项奥斯卡金像奖。

在《百万美元宝贝》之后，哈吉斯参与了 2004 年的电影《撞车》。哈吉斯自己想出了这部电影的故事，然后编剧并导演了这部电影，这使他能够更好地控制自己的工作。《撞车》是他第一次担任大型故事片的导演。上映后非常积极，此后对《撞车》的评论反响两极分化，尽管罗杰·艾伯特称其为 2005 年最佳电影。

《撞车》获得了奥斯卡最佳影片和最佳导演奖提名，此外还获得了其他四项奥斯卡金像奖提名。哈吉斯凭借这部电影获得了两项奥斯卡奖：最佳影片（作为制片人）和最佳编剧奖。凭借《百万美元宝贝》和《撞车》，哈吉斯成为第一位连续两年获得奥斯卡最佳影片奖的人。

哈吉斯说，他写《崩溃》是为了“打击自由主义者”，认为他的自由主义者同胞对种族和种族主义的本质并不诚实，因为他们认为大多数种族问题已经在美国社会得到解决。