Leiningen served as a Bavarian lieutenant general, before he briefly played an important role in German politics as the first Prime Minister of the Provisorische Zentralgewalt government formed by the Frankfurt Parliament in 1848.

Prince Emich Carl had received the Principality of Leiningen during the German mediatisation (Reichsdeputationshauptschluss) in 1803, as a compensation for the lost Hardenburg estates in the Palatinate occupied by French revolutionary troops, and took his residence at the secularised Amorbach Abbey.

On 11 July 1818, his widowed mother married Prince Edward Augustus, Duke of Kent and Strathearn, the fourth son of King George III of the United Kingdom, at Kew Palace, Surrey.

On 20 April 1842, he and 20 other noblemen gathered at Biebrich Palace, where they established the Adelsverein to organize the settlement of German emigrants in Texas; Karl was elected president of the society.

He advocated the implementation of parliamentarism and openly criticized aristocracy's privileges; therefore, he was appointed Prime Minister of Revolutionary Germany by Regent (Reichsverweser) Archduke John of Austria on 6 August 1848.

His cabinet initially could rely on a liberal and left-wing majority in the newly established Frankfurt Parliament, however, as early as on 5 September, he resigned over the Schleswig-Holstein Question when in the First Schleswig War King Frederick William IV of Prussia unilaterally signed an armistice with Denmark at Malmö.