His reign was marked by an unsuccessful Turkish attempt to expel him and by the first treaty between Crimea and the Zaporozhian Cossacks.
A few months later Mehmed's son Saadet II Giray invaded, made himself khan and was driven out.
Around 1594, during the reign of Gazi II Giray, Mehmed arrived in Crimea along with his two brothers and mother.
A few weeks later Mehmed's brother Shahin arrived from Circassia and was made nureddin (third in rank after khan and kalga).
Hearing that the Turkish troops were leaving they invaded Crimea and were soundly defeated by the remaining janissaries.
In February 1618 Mehmed escaped, was captured on the Bulgarian coast and exiled to the isle of Rhodes where he was well-treated.
Mehmed led the whole Crimean army west and somehow talked Khan Temir into withdrawing east to the Syut-Su River (location?).
In revenge the Mansur bey raided Poland and took so many captives that prices on the slave market collapsed.
Mehmed proved a stronger ruler than his predecessor Janibek, which caused hostility among the nobility.
A group of Zaporozhians who had been captured after their boats were washed up on shore by a storm were offered their freedom if they would fight for Mehmed.
On 11 August Rejeb Pasha marched out of Kaffa with about 10000 soldiers and the cannon he had removed from the boats and fortress walls.
Mehmed carefully held back since entering Kaffa would be an invasion of Turkish territory.
Shahin and the Poles: Mehmed was careful not to antagonize the Turks unnecessarily, but his brother was more aggressive.
Starting in January 1626 a Crimean-Budjak army plundered some 200 villages in Volhynia and Galicia and withdrew before Koniecpolski and Stefan Chmielecki could bring up troops.
A later raid by Khan Temir and the nureddin Azamat Giray was defeated by Chmielecki and the Cossack Mykhailo Doroshenko.
Temir's nobles convinced him that their position was untenable, so the Budjak Horde moved back east after burning everything they could not carry.
In early 1627 Mehmed led 10000 Crimeans and Budjaks east to deal with some Besleney who had stopped paying tribute.
As he left some Budjak mirzas slipped away and murdered Mehmed's father-in-law who in 1622 had killed the uncle of Khan Temir.
Mehmed assumed that this could not have happened without the consent of Khan Temir, so he sent a messenger to Shahin ordering him to be arrested.
Janibek appeared offering to lead the Crimean army to Persia if he were made khan.
In March 1628 Shahin set out toward Poland while in fact planning to attack Khan Temir.
He attacked, the Budjaks fled and he chased them into the woods where he found himself surrounded by Khan Temir.
Doroshenko was killed, Khan Temir was wounded, Azamat Giray fled to Akkerman and the brothers left Chufut-Kale to meet their new friends.
Shahin wanted to storm the town before a Turkish fleet arrived, but Mehmed held him back, not wishing to provoke the Turks.
Around 15 November they approached Or-Kapi, but found that Janibek, his brother Devlet and Khan Temir had gotten there a few hours before.
They landed on the western coast, went through the forests and captured the ancient fort of Mangup-Kale which was weakly defended and used by the khan as a treasury.
Janibek was worried because he was outnumbered, but scouts reported that there were rows of carts with barrels supplying the enemy.
The battle was inconclusive, but toward the end of the day thirst began to take effect and many of the Cossacks wanted to go home.
He slipped through the lines and told Khan Temir's men that he would surrender in exchange for his life.
Mehmed told the Cossacks that the approaching force were allies that he had called in, so the Zaporozhians opened their lines.