Paul's writings, and even those of Luke who wrote Acts, do not definitely state that the apostle was ever a member of the Sanhedrin. That said, there are a few hidden New Testament clues that might shed some light on this fascinating subject.
Who Were They?
The Sanhedrin, known as "the council" (Matthew 26:59, Mark 14:55), was the widely recognized first century religious authority over the Jews. It was composed of seventy members plus the High Priest. The Pharisees (e.g. Nicodemus, John 3:1 - 10) as well as the Sadducees, scribes, elders and possibly others, were represented on the council.
According to the AMG Concise Bible Dictionary, the Roman Empire gave the Sanhedrin authority to arrest, judge and punish Jewish people for offenses primarily relating to religious law (Mark 14:43, Acts 5:17 - 21, 6:11 - 15). The Sanhedrin was the group who insured Jesus would be crucified by the Romans. They were also the driving force in persecuting the early New Testament church (Acts 4, 5:17 - 41).
A Key Verse
The one verse that offers a clue regarding Apostle Paul's pre-conversion Sanhedrin membership is found in Acts 26. Paul, while a Roman prisoner in Caesarea, is given an opportunity to explain his case to King Agrippa. His opening statements introducing himself to Agrippa included the following.
Which I (Paul) also did in Jerusalem; and many of the saints I shut up in prisons, having received authority from the chief priests; and when they were put to death, I gave my full consent (kataphero psephos) against them (Acts 26:10, HBFV).
The New International Version, like other Bibles such as the NKJV and NASB, translates the Greek words kataphero psephos used above as the following.
. . . I cast my vote against them (Acts 26:10, NIV).
The Greek word kataphero (Strong's Concordance #G2702) means to cast a vote. The word psephos (#5586), however, refers to a small stone, usually colored either white or black, used in voting to determine whether someone was either innocent or guilty.
The only other usage of psephos in the Bible is in Revelation. It is used by Jesus to symbolically represent that those who overcome will not only be considered guiltless, they will be given a new name underscoring their unique relationship with him.
To the one who overcomes I will give the right to eat of the hidden manna; and I will give him a white stone (psephos), and on the stone (psephos) a new name written, which no one knows except the one who receives it (Revelation 2:17).
Some Bible commentaries say that Apostle Paul's statement in Acts 26:10 does not mean he was a member of the Sanhedrin. They state it merely refers to his personal pre-conversion approval of killing Christians for their faith. If that were the case, however, then only the word kataphero would have been used.
"The ancient Greeks used white pebbles for acquittal (Revelation 2:17), black ones for condemnation as here (in Acts 26:10) . . . Paul's phrase (not found elsewhere) is more vivid than the usual kataphero for voting. They literally cast the pebbles into the urn . . . If Paul's language is taken literally here, he was a member of the Sanhedrin . . ." (Word Pictures in the New Testament).
The use of the unique phrase kataphero psephos lends credence to the belief that Apostle Paul was likely, at one time, a Sanhedrin member.
"The Greek of "my voice" (found in King James version of Acts 26:10) means 'the vote of a judge,' and establishes the fact that at the time of the death of Stephen, Paul, though so young a man, was a member of the Sanhedrin (Commentary on the Holy Bible: The One Volume Bible Commentary).
A Reasonable Conclusion
Paul's pre-conversion life offers circumstantial evidence that membership in the Sanhedrin, even at a young age, was entirely possible and probable.
Paul's father was a well-respected Pharisee who had the Jerusalem connections necessary to have his son taught by the famous Gamaliel (Acts 22:3, 23:6). Gamaliel was a well-known Pharisaic teacher and Sanhedrin member (5:34) who was the first one honored by the Jews with the title of "Rabban" (1906 Jewish Encyclopedia article on Gamaliel I).
Apostle Paul, after his religious schooling, became a Pharisee who surpassed many others in their zeal for Judaism.
And I was advancing in Judaism far beyond many of my contemporaries in my own nation, being more abundantly zealous for the traditions of my fathers (Galatians 1:14, HBFV).
Apostle Paul's (Saul's) access to both chief priests (Acts 26:10) and to the temple's High Priest (Acts 9:1 - 2) suggests he was both known and highly trusted by the most powerful Jewish religious leaders. He was also given a great deal of authority at a young age, able to go house to house in Jerusalem to find and arrest those who professed faith in Christ (Acts 8:3). He was even allowed to travel to foreign cities to hunt down believers (26:11).
It is therefore not hard to imagine that Apostle Paul (Saul), given his connections, education and zealous willingness to persecute hated Christians, would be made a Sanhedrin member. If he was a member then he was likely also married.
"Paul himself must have been married, otherwise he could not have been a member of the Jewish Sanhedrin." (Wiersbe Expository Outlines).