LOS ANGELES — Christopher J. Dodd, ending an uneven six-year tenure, will step down in September as chief executive of the Motion Picture Association of America and will be succeeded as Hollywood’s top lobbyist by Charles H. Rivkin, a former United States ambassador and assistant secretary of state.

In announcing the change on Friday, the chiefs of Hollywood’s six major studios lavished praise on Mr. Dodd, 72, who took over the movie organization after serving as a Democratic senator from Connecticut for three decades.

“He helped create a fully integrated, global online antipiracy operation,” Stacey Snider, chief executive of 20th Century Fox, said in a statement. Mr. Dodd was also credited with helping to gain distribution for Hollywood movies in China.

China is likely to be Mr. Rivkin’s initial focus. The studios are working to renegotiate Chinese rules that limit how many Hollywood movies are shown in the country each year. “If you ask these guys what their three major goals are, they will tell you China, China, China,” Mr. Dodd said in a telephone interview, referring to studio executives.

But Mr. Dodd, who plans to stay with the association until the end of the year to help with the transition, will also be remembered for two moments when the organization faltered.

A major push in 2012 by copyright holders — including those represented by Mr. Dodd — for a tough federal law to control foreign online piracy collapsed in the face of stiff resistance from Google and other technology giants. Mr. Dodd had just taken over as the association’s chief and had to adhere to legislation that forbade him from personally lobbying Congress for two years after leaving office, but the failure of the law, the Stop Online Piracy Act, became a black eye nonetheless.

Mr. Dodd was also faulted for his handling of a 2014 cyberattack on Sony Pictures. As hackers exposed thousands of emails and internal documents, Sony was left unassisted for a time, with Mr. Dodd unable to persuade executives at Hollywood’s other major studios to offer public support. Sony threatened to withdraw from the organization over the episode, but ultimately remained a member.

Sounding jovial and joking about no longer having to spar with the executive Harvey Weinstein over film ratings, Mr. Dodd said he was proud of his track record, particularly regarding online piracy. “I wanted to leave the organization in a better place than I found it,” he said, “and I believe that is the case.”

Mr. Rivkin, 55, is a familiar face in show business. Before his government career, he ran the Jim Henson Company, the ancestral home of the Muppets. He was also the chief executive of Wildbrain Entertainment, known for producing the hit series “Yo Gabba Gabba!” He was named United States ambassador to France in 2009 and later became the top State Department official for business and economic affairs under President Barack Obama.

Despite his experience, Mr. Rivkin faces an arduous task at the association. The organization, which runs Hollywood’s movie ratings system, has been weakened by budget cuts. All of the member studios are embedded within global media conglomerates, which employ their own lobbyists and often have competing interests. And when it comes to influence in Washington, Hollywood has been surpassed by Silicon Valley.

“In today’s digital world, the mission and function of the M.P.A.A. are more important than ever,” Mr. Rivkin said in a statement. “Chris has done a great job, and I’m excited to work with the member studios to continue to expand the global market for entertainment content, while ensuring creators’ rights are respected around the world.”