Born 1610±. Came from a rather wealthy family settled in Greater Poland, of good standing in the provinces of Łęczyca and Sieradz. Was related to the Leszczynski family, having a great influence in the late 1640s.
As a young man he served in a Cossack regiment as a Towarzysz. He participated in battles against the Swedes (1626-1629). Under the command of Hetman Stanislaw Koniecpolski he took part in the suppression of the Taras Fiedorowicz Cossack uprising in 1630-1631.
Was a veteran of many military expeditions, including campaigns against Moscow and the Swedes.
He was a bearer of the Lubicz coat of arms.
In 1648 he was Chamberlain of Poznań.
From 20 February 1659 – 18 August 1681 he was Castellan of Gniezno.
From 1655-1659 he was Castellan of Łęczyca
From 23 July - 17 August 1652 he was 74th Marshal of the Polish–Lithuanian Commonwealth. This term was cut short, incidentally ending four days before the wedding of Hetman Bohdan Khmelnytsky's son Tymofiy Khmelnytsky, to Ruxandra Lupu the daughter of Basil Lupu, Prince of Moldova. (August 21, 1652)
He was part of "Obronie Złotej Wolności” movement (Defense of "Golden Liberty") that opposed the war plans of the deceased king Władysław IV Vasa. Sielski supported equal rights for all subjects of the Polish-Lithuanian Commonwealth, including Cossacks.
During the Khmelnytsky Uprising he was one of the most prominent members of the negotiations. The Sejm resolution of 22 July 1648 chose Aleksander Sielski (Chamberlain of Poznań), Adam Kisiel (Voivode of Bracław), Franciszek Dubrawski (Chamberlain of Przemyśl) and Teodor Obuchowicz (chamberlain of Mazyr), to negotiate with Bohdan Khmelnytsky. The negotiations ended in failure in February 1649.
Aleksander Sielski fought in the Second Northern War - part of a series of wars known as The Deluge. During the Swedish invasion Sielski remained faithful to King John II Casimir. At the end of September Sielski came to Nowy Sacz, where on 30 September 1655 participated in the council of the senate at the king's side, accompanied by King John II Casimir on the way to Głogówek, where the procession arrived on 17 October. Sielski took part in the council of the Senate with the participation of the king in Krosno, continued in Lancut in January 1656.
Sielski was among the first senators to sign - with representatives of the army - an act to confederate the states of the crown to launch an offensive against the Swedes in order to save Poland. At the end of January Sielski, together with Stanislaw Witowski (Castellan of Sandomierz), was sent to help Stefan Czarniecki, the organizing winter expedition in Sandomierz and Lublin, with the task to create and mobilise armies of the Polish nobility with which to fight the Swedes.
In February Sielski fought in the Battle of Gołąb. In March Sielski rendezvoused with the king in Lviv, and then accompanied him in a march to Warsaw, where he witnessed the surrender of Swedish Field Marshal Arvid Wittenberg. Arvid was imprison in Zamość where he later died in 1657. Sielski held the position of administrator of the treasury, tasked with raising funds to reorganize and pay the crown's army during the Battle of Warsaw.
In Warsaw Sielski was accompanied by the queen, and then together with the court he went to Lublin. In August 1657 Sielski went as a representative of the king to accept the surrender of the Swedes. The act of surrender was signed by Swedish General Paul Würtz in a camp near Kraków, and in September Sielski filed the signature on the ratification of the Treaty of Roskilde with Denmark. Sielski participated in parliament, as a deputy to the Court treasury in Radom and Rawa.