The Kennedys have held US congressional seats, the Presidency and the public’s imagination for more than 60 years.

That era ends when Patrick Kennedy of Rhode Island vacates his US House seat next month, leaving a city council post in California as the family’s sole remaining political holding.

The son of the late Massachusetts Senator Edward Kennedy says he has no qualms about walking away from politics. His departure marks the first time in 63 years that a Kennedy will not be serving in elected office in Washington.

The 43-year-old Kennedy says in his family, “the legacy was always public service, and that didn’t necessarily mean public office”.

Mr Kennedy plans to continue the tradition by championing a national effort to boost brain research. He recited a long list of Kennedy family members who have spurned politics and chosen lives as activists promoting issues such as the environment, human rights and women’s issues.

Mr Kennedy hopes to inject the same urgency into his brain research work that his late uncle, President John F. Kennedy, inspired during the 1960s with his challenge to Americans to put a man on the moon.

But Mr Kennedy’s exit from the US capital marks a bittersweet turn for one of America’s most powerful and prominent political families, a family that has seen its influence in Washington fade in recent years as its younger generations have largely shunned public office.

“It is a milestone,” said Allan Lichtman, an American University history professor. “Frankly, it’s not as if there’s a new generation of Kennedys ready to move into public life in a major way.”

Politics was the family business, the lifeblood of a dynasty that often dominated the public stage with its triumphs, as well as its personal traumas.

John F. Kennedy arrived as a young congressman in 1947, later capturing the White House and leaving an outsized stamp on the nation’s history. Robert F. Kennedy’s presidential ambitions were snuffed out by an assassin’s bullet, but he inspired generations of activists. Edward M. Kennedy was seen as one of the most influential senators in US history.

Now it is Bobby Shriver as the lone family member left in politics, serving as a city councillor in the seaside Los Angeles suburb of Santa Monica.

Mr Shriver said he first got into city politics not because of his family’s legacy, but because of citations he and neighbours received over the height of their hedges.

He has served as mayor and made helping homeless veterans a priority. He has long been an activist, working with U2 singer Bono to aid countries in Africa.

Politics is not the only way to make a difference, Mr Shriver said, recalling his mother, the late Eunice Kennedy Shriver, who founded the Special Olympics. Her efforts were inspired by the struggles of her mentally disabled sister, Rosemary, who spent most of her life in an institution after a lobotomy left her permanently incapacitated.

That sister of JFK played a big role changing America’s view of the mentally disabled from that of institutionalised patients to friends, neighbours and athletes.

“My mum changed the world without being elected to office,” Bobby Shriver said. “Period. End of story.”

Patrick Kennedy agreed her legacy may be his famed family’s most impressive. “In the totality of history, she may go down as the one who made the biggest difference,” he said.

Mr Kennedy is a leading voice on mental health issues. He championed a landmark bill that Congress passed requiring insurance companies to treat mental health on an equal basis with physical illnesses.

He has often spoken candidly about his personal struggles with depression and substance abuse. Mr Kennedy has been treated for substance abuse since crashing his car outside the Capitol in 2006, and he has struggled with alcoholism and drug addiction for much of his life.

Mr Kennedy will highlight the need for expanding brain research with a conference in Boston in May on the 50th anniversary of JFK’s moonshot challenge.

He recalled that the treatment his father received helped prolong his battle with brain cancer.

“My dad was given an extra year of life because the science was good,” Mr Kennedy said. “Against initial expectations, he had a much longer time, and that was meaningful time, at least in our relationship. So that’s personal.”

Five months after his father’s death in 2009, Mr Kennedy announced he would not seek a ninth term.

As he leaves Congress, there is speculation about who could emerge, if anyone, to revive the family’s political legacy in Washington.

Most recent Kennedy political bids have failed.

Darrell West, vice president and director of governance studies at the Brookings Institution, said it has become more difficult over the years for Kennedy family members to extend their brand of liberalism outside the US northeast.

“As the country has grown more conservative,” Mr West said, “it’s been harder for the Kennedys to win elective office.”

The Kennedy family

• The Kennedy family’s political involvement has revolved around the Democratic Party.

• Following the 1960 election of US President John F. Kennedy, he and his two younger brothers, Robert F. Kennedy and Edward M. Kennedy, soon all held prominent positions in the federal government.

• From 1947, when John F. Kennedy was first elected to the US Congress, to next year, when Patrick J. Kennedy will leave Congress, there has been a 64-year run of a Kennedy family member.

• The Kennedys have suffered a series of tragedies, including the assassination of brothers John and Robert, the controversial Chappaquiddick incident and four airplane crashes.