Members of the armed services accompanied the hearse as it arrived at the library in the Simi Valley city.
The funeral takes place on Friday.
Nancy Reagan died at the age of 94 last Sunday. Her 52-year marriage to Ronald Reagan was once described as the US presidency's greatest love affair.
Mrs Reagan, who died of congestive heart failure in Los Angeles, was praised by President Barack Obama's family and other senior US politicians and foreign leaders.
From 1981-89 she was one of the most influential first ladies in US history, initially criticised for an expensive renovation of the White House, but later becoming a much-loved figure.
Nancy Reagan: Her husband's greatest supporter
Praise for 'devoted' First Lady
Mrs Reagan will be buried next to her husband at the presidential library.
Like her husband, Mrs Reagan was a former Hollywood performer who made it all the way to the White House.
As Nancy Davis, she was an actress during the 1940s and 1950s and married Mr Reagan, a prominent film actor, in 1952.
She served as first lady of California during her husband's stint as California governor from 1967 to 1975 before moving into the White House after his decisive victory over Democrat President Jimmy Carter in 1980.
As first lady, she sought to emulate the style of one of her predecessors, Jackie Kennedy.
To this end, she extensively redecorated the White House, and accepted designer dresses worth $1m (Â£703,000) and a 4,732-piece set of china worth $209,000.
But this spending spree provoked an outcry from some outraged by what they saw as profligacy and waste while millions of Americans were losing their jobs.
Public opinion was also swayed by accusations that Mrs Reagan had a frosty personality, often consulted astrologers, and ordered the dismissal of White House chief of staff Donald Regan in 1987.
"I see the first lady as another means to keep a president from becoming isolated," she once said.
Mrs Reagan's best-known project as first lady was the anti-drugs "Just Say No" campaign.
After her husband died of Alzheimer's disease in 2004, she became a champion for Alzheimer's patients, raising millions of dollars for research and breaking with fellow conservative Republicans to argue for stem cell research.

The error was made by RingGo payment system, which is managed by City of Edinburgh Council's contractor NSL.
Motorists who tried to use RingGo on Tuesday will have their tickets cancelled.
The council said it clearly stated on its website and parking machines that 27 December was a paid parking day.
The council said about 300 motorists had been affected by the blunder.
A council spokeswoman said: "Details relating to parking restrictions during the festive period are advertised very clearly through our website and social media and are provided on ticket machines throughout the city.
"We are aware, however, that there was an error for a time with the RingGo payment service managed by our contracted partner NSL.
"We are liaising with NSL to ensure that RingGo users who were issued with a parking ticket, due to the system failure on Tuesday, have their tickets cancelled."
An NSL spokesman said: "NSL has confirmed that parking restrictions and charges in the city of Edinburgh do apply on Tuesday 27 December and Monday 2 January as stated on all Pay and Display machines and on the City Of Edinburgh Council's website.
"Full enforcement procedures apply on both dates in order to maintain safe and fair access for anyone visiting the shops and facilities in and around the city centre.
"The company has confirmed that it has been made aware of an incorrect instruction on the RingGo app and RingGo website and has asked for the instructions to be amended as a matter of urgency."
The coffin of former US First Lady Nancy Reagan has been placed in the Ronald Reagan Presidential Library in California for public viewing.
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Hundreds of drivers have been issued parking tickets after an app wrongly told them they could park for free in Edinburgh on the day after Boxing Day.