Joining a magnate conspiracy in 1401, he played a key role in the arrest of the king, but later was pardoned and retained his political influence until his death.

Simon's grandfather was Thomas I Szécsényi, who rose to prominence during King Charles I's war against the oligarchs and received numerous grants of land thereafter.

The Szécsényi brothers were one of the earliest domestic partisans of Sigismund of Luxembourg, who arrived to Hungary to validate the marriage agreement with Queen Mary.

[5] In official documents, Szécsényi first appeared as count on 27 February 1391, when issued a diploma in Gyulafehérvár (today Alba Iulia, Romania).

[9] As Sigismund prepared a war against the Ottoman Empire and its allies Wallachia and Moldavia since that year, which sparked into the so-called Crusade of Nicopolis, he was far away from the royal court for a long time.

[9] After the disastrous defeat at Nicopolis, Szécsényi took an active in role in assassination of his former ally, the once powerful Stephen Lackfi, who was massacred by the followers of the Kanizsai League along with his kinship in the Bloody Sabor of Križevci on 27 February 1397.

[11] In the upcoming years, the relationship increasingly worsened between the Hungarian barons (de facto led by Archbishop Kanizsai) and Sigismund, especially due to the latter's favoritism towards his foreign courtiers, such as Pipo of Ozora, Stibor of Stiboricz, Hermann II, Count of Celje and Bishop Eberhard Albeni.

The Siklós League, led by Nicholas II Garai (Szécsényi's brother-in-law) and Hermann of Celje, took the power over the royal court after the 1401 conspiracy.

[1] For his participation in the counterinsurgency, Szécsényi was made Master of the doorkeepers (thus also Marshal of the Royal Court), holding the office from 1403 to 1409 (there was a short interruption between July 1405 and January 1406, for unknown reasons).

Feeling the nearness of his death, he concluded an inheritance contract with his nephew Ladislaus (II) in the presence of Judge royal Simon Rozgonyi on 9 December 1411 in Letkés.

Accordingly, his son Nicholas would be the sole heir of Salgó Castle, which was acquired by Simon decades earlier, while Tapolcsány (today Topoľčany, Slovakia) belonged to Frank's branch.