Their appointments are effective on Oct 1, but still pending His Majesty the King's endorsement.
The Defence Committee unanimously agreed on both appointments during a one-hour meeting chaired by Defence Minister Sukumpol Suwanatat, according to the source.
Gen Thanongsak is an assistant army chief and was backed for his new job by ACM Sukumpol. ACM Prajin is an assistant air force commander and was favoured by air force chief Itthaporn Subhawong to succeed him when he retires on Sept 30.
ACM Itthaporn said the one-hour meeting went smoothly with no one objecting the nomination for defence secretary.
The source said all armed forces chiefs and the supreme commander had also put their signatures to documents officially approving the defence reshuffle list. The minister will soon send it to Prime Minister Yingluck Shinawatra, who will forward it to His Majesty the King for royal approval.
ACM Sukumpol said after the meeting that there was no conflict in the selections and all the council members had approved all the military reshuffle list with no need for a vote.
The Defence Council meeting was in the spotlight following the public rift between the minister and retiring permanent secretary for defence Sathian Permthong-in, who backed his deputy, Gen Chatree Thatti, to be his successor.
The conflict led to the decision by ACM Sukumpol to abruptly move Gen Sathian to an inactive post at the minister's office. He appointed Withawat Ratchatnant acting permanent secretary for defence.
Gen Sathian was unhappy with the sudden transfer and has taken his case to the Administrative Court, seeking justice.
Gen Withawat said the committee decided before the meeting that he and Gen Sathian would not be allowed to make the decision on this year's reshuffle list, for fear of possible legal problems after the court’s decision.
The court had not made a decision at the time of today's meeting.