Pornpetch is a legal advisor to the military junta, or the National Council for Peace and Order (NCPO), Xinhua reported.
Former Senate speaker Surachai Liengboonlertchai was selected as first vice president and former deputy Senate speaker Peerasak Porchit, the second vice president at the session which was by 193 members.
A royally-endorsed list of 200 NLA members was announced late July, three of whom were later found ineligible for the post.
NLA members are now expected to nominate an interim prime minister to be appointed by the king.
The interim PM will select up to 35 cabinet members.
NCPO chief Prayuth Chan-ocha is cosidered to be the top candidate for the PM post.