The 10km Red Line extension that joins Hollywood to the rapidly growing northern suburbs of Los Angeles known as the San Fernando Valley has taken 13 years to build and cost more than $4.5bn.
Engineers found themselves dealing with flooding, soil contamination and even methane gas. On one occasion they had to stop work after part of the Hollywood Boulevard started to subside.
The subway's operators hope it will persuade commuters to abandon cars.
'13-line freeway'
Metropolitan Transportation Authority spokesman Gary Wosk said: "If used to capacity, the subway would be the equivalent of a 13-lane freeway. But it is hard to reverse the mind-set of the car that has been predominant over the past 50 years."
So far the Red Line has attracted fewer than a quarter of the predicted users.
But only about 62,000 people use the existing system on an average weekday, far less than the 290,000 projected.
The Red Line is six miles (9.7km) shorter than originally planned. At $259m a mile, it is seen as perhaps the most expensive underground rail link in US history.
Free travel
Passengers travelled free on the extension running under the Hollywood Hills on Saturday to commemorate its opening.
The Red Line had long been derided as 'the subway to nowhere'. The project stopped short of the Los Angeles International Airport and does not extend to the wealthy district of Beverley Hills.
Transportation officials are nonetheless hoping that rising petrol prices will finally convince people to leave their cars at home.
According to the BBC's correspondent David Willis, pampered by valet parking and luxury limousine services, people prefer to spend hours on some of the most gridlocked freeway systems in America in the belief, it would seem, that it is better to travel than to arrive.
Los Angeles residents voted in 1998 to ban further use of tax money for subway construction, and our correspondent says officials will have to look elsewhere to solve the city's growing transportation problems.