[2] On 5 June 2014, he was named Shadow Deputy Minister of the Justice and Correctional Services portfolios by the DA parliamentary leader, Mmusi Maimane.

[4] During his first term as an MP, he served as an Alternate Member of the Portfolio Committee on Justice and Constitutional Development.

[3] He was an Alternate Member of the Ad Hoc Committee to nominate a person for appointment of Public Protector between May 2016 and August 2016.

[6] Horn was appointed the DA's additional national spokesperson on 21 April 2023, following Cilliers Brink's election as Tshwane mayor the previous month.

[8] On 14 November 2020, Horn was elected to succeed Annelie Lotriet as the provincial chairperson of the DA in the Free State.