The American was the sole contender for the fourth position of IOC vice-president to replace Australia’s John Coates, who has come to the end of his eight-year term.
A total of 80 voters were eligible to take part in the election, with 60 voting in favour of DeFrantz.
There were 16 votes against and four abstentions.
DeFrantz, a former rower, previously served as IOC vice-president from 1997 to 2001.
The 64-year-old, a member of the IOC Executive Board for the past four years, thanked the members for their support of her candidacy and decision to award Los Angeles the 2028 Olympic and Paralympic Games.
Los Angeles 2028 have hailed the election of "inspirational" DeFrantz, who has been serving as their senior advisor for legacy.
"Anita has been an inspirational leader of the LA 2028 team, and we are extremely proud to see her elected as vice president for the second time in what has been a magnificent career,” Casey Wasserman, the chairman of Los Angeles 2028, said.
"She is a great role model for the younger generation and we could not wish for a better ambassador for the Olympic Movement in our country.
"Due to Anita’s ground-breaking work, the Los Angeles 1984 Games set a new standard for putting Olympic Legacy into action
“The achievements of the LA84 Foundation under Anita’s leadership will serve as a blueprint and an inspiration to us as we look to kick-start a fresh Olympic and Paralympic legacy in the build-up to Los Angeles’ first Games in a generation."
DeFrantz is only one of three IOC vice-presidents to be women.
Sweden’s Gunilla Lindberg followed DeFrantz from 2004 until 2008 and then Nawal El Moutawakel of Morocco served in the position from 2012 to 2016.
DeFrantz will serve an initial four-year term.
Coates, who leaves the Executive Board having served his maximum term, saw his IOC membership status changed.
His membership is no longer related to him being President of the Australian Olympic Committee.
Fiji's Robin Mitchell was confirmed as one of three members to join the ruling Executive Board.
He was allowed to run unopposed for the position as the representative for the Association of National Olympic Committees (ANOC).
The 71-year-old is an ANOC vice-president and was re-elected for a third term as President of the Oceania National Olympic Committees earlier this year.
He has served as an IOC member since 1994, so is not constrained by the age limit of 70 introduced for members who joined after 1999.
Mitchell got 76 votes with four against.
The position opened following the resignation of Ireland’s Patrick Hickey, who resigned last week as he seeks to clear his name from charges of theft, tax evasion, money-laundering and criminal association following his arrest during Rio 2016.
Hickey still hopes to return as an IOC member.
Hoevertsz was elected to the second position on the Executive Board after winning in the fourth round of voting against Switzerland’s Denis Oswald with 46 votes to 31.
The 53-year-old, who represented Aruba at the 1984 Olympics in synchronised swimming, thanked the IOC membership for having trust in a woman from a small National Olympic Committee to take up the Executive Board post.
Oswald took the final place on the IOC Executive Board, with the second round victory in the next sequence of voting.
The Swiss received 40 votes as he defeated Nigeria's Habu Gumel and Hungary's Pal Schmitt, who got 25 and 11 votes respectively.
It had been widely believed that Gumel had been the preferred choice of IOC President Thomas Bach to be elected to the Executive Board as there is currently no representative from Africa.
Peru's Ivan Dibós was eliminated in the first round of voting.
Oswald has been chairing one of two IOC Commissions which have been investigating the Russian doping crisis.
The election sees Oswald return to the Executive Board, with the 70-year-old having previously done so from 2000 to 2012.
During that period he chaired the IOC Coordination Commissions for Athens 2004 and London 2012.