  The aim of this thesis is the completion of the NLO automated framework of
the MC program WHIZARD, accounting for NLO corrections in the full SM for cross
sections and distributions of processes at hadron and lepton colliders.
Specifically, it builds on the implemented FKS subtraction scheme for NLO QCD
calculations, and extends it to automated NLO EW and QCD-EW mixed corrections.
To that end, the implemented FKS scheme is generalised to systematically
subtract QED and QCD infrared (IR) divergences in mixed coupling expansions.
The automated computation of NLO contributions is validated for a set of
benchmark processes at the LHC, including e.~g. $t\bar{t}~(+H/W/Z)$ production.
Cross-checks for $e^+e^-$ processes likewise show that WHIZARD can be used for
predictions at lepton colliders including fixed $\mathcal{O}(\alpha)$
corrections. This framework is applied to the study of NLO EW cross sections
and differential distributions for multi-boson processes at a future multi-TeV
muon collider. For some high-energy lepton-collider setups, tiny initial-state
masses jeopardise the reliability of fixed-order expansions of observables in
QED perturbation theory. In order to recover meaningful predictions for NLO EW
observables, QED parton-distribution functions (PDFs) must be applied
guaranteeing a universal treatment of collinear ISR effects. This however
causes computational challenges for MC integration measures. Methods to cope
with these difficulties are presented in this thesis. With the automation of
NLO EW corrections for hadron and lepton collision processes, WHIZARD offers a
powerful tool for EW precision studies at current and future colliders and
provides the desired accuracy level of predictions for new physics searches.
