Assembly Members also elected Green Party AM Darren Johnson as Deputy Chair.
Both AMs held the positions during some of the previous City Hall term.
Arnold said she was “honoured to be serving a third term as Chair of the London Assembly” and thanked colleagues for their “confidence”.
She added: “We have just fought a robust election but now is the time for all Assembly Members to join together as we work to improve the quality of life in London across the full scope of the Greater London Authority’s responsibilities.
“The Mayor has significant new powers to address regeneration and housing need in the Capital. The Assembly will be watching closely to see that he delivers on his election promises to Londoners, and does so in an efficient and effective manner.”
The elections follow a deal between the Labour, Green and Liberal Democrat groups on the Assembly to share the majority of posts, including the Chairs and Deputy Chairs of committees, between them.
Similar agreements have existed since the Assembly was formed in 2000.
During Ken Livingstone’s second term Conservatives and LibDem AMs blocked Labour members from holding the major posts.
The three groups have defended their agreement as a counter-balance to the jobs Mayor Boris Johnson has given Tory AMs including the Chairmanship of LFEPA to James Cleverly and Kit Malthouse’s appointment as deputy mayor for Business.
Len Duvall AM said: “The Labour, Liberal Democrat, and Green groups have agreed to work together to scrutinise the Conservative Mayor of London, Boris Johnson.”
Green Party AM Jenny Jones added: “The Assembly is elected by Londoners to hold the Mayor to account and the most effective way of doing that is by including all party groups in operational arrangements while also ensuring the Mayor is fully held to account with the non-mayoral parties taking the lead.”
During Friday’s meeting Conservative group leader Andrew Boff AM said the deal “disenfranchised the 700,000 Londoners who voted Conservative” in last week’s election.
The meeting also elected the following Chairs and deputies to the Assembly’s scrutiny committees:
Audit Panel (4 Members)
Chair: John Biggs AM
Deputy Chair: Roger Evans AM
Budget and Performance Committee (9 Members)
Chair: John Biggs AM
Deputy Chair: Stephen Knight AM
Confirmation Hearings Committee (9 Members)
Chair and Deputy Chair to be appointed at meetings.
Economy Committee (9 Members)
Chair: Andrew Dismore AM
Deputy Chair: Stephen Knight AM
GLA Oversight Committee (9 Members)
Chair: Len Duvall AM
Deputy Chair: Jennette Arnold AM
Health and Environment Committee (9 Members)
Chair: Murad Qureshi AM
Deputy Chair: Jenny Jones AM
Housing and Regeneration Committee (12 Members)
Chair: Len Duvall AM
Deputy Chair: Darren Johnson AM
Planning Committee (9 Members)
Chair: Nicky Gavron AM
Deputy Chair: to be elected at first meeting
Police and Crime Committee (12 Members)
Chair: Joanne McCartney AM
Deputy Chair: Caroline Pidgeon AM
Deputy Chair: Jenny Jones AM
Standards Committee (5, plus 6 independent Members)
Chair and Deputy Chair to be elected at first meeting.
Transport Committee (12 Members)
Chair: Caroline Pidgeon AM
Deputy Chair: Val Shawcross AM
