Answer
Lord Castlereagh, Londonderry's half-brother, played a pivotal role in launching his diplomatic career. Castlereagh, a prominent politician and diplomat, recognized Londonderry's potential and became his mentor. Through Castlereagh's guidance and support, Londonderry gained valuable experience and rose to prominence in the diplomatic sphere. Castlereagh's influence and network connections proved instrumental in shaping Londonderry's successful diplomatic trajectory.