Gold‐catalyzed imination/Mannich reaction cascades on readily available 3‐en‐1‐ynamides enable the diastereoselective synthesis of 1,5‐iminoamino compounds in a one‐pot operation. The reactions work well with diversified 3‐en‐1‐ynamides, aldehydes and anilines with good to excellent diastereoselectivities. Our control experiments indicate gold‐catalyzed aminations of 3‐en‐1‐ynamides to yield α‐imino allylgold intermediates that react subsequently with iminiums to implement the Mannich reactions stereoselectively. The reactions proceed with anti‐selectivity for those 3‐en‐1‐ynamides bearing acyclic alkenes whereas syn‐selectivity is seen for their cycloalkene‐based analogues. An open‐transition state can satisfactorily rationalize these observed stereoselectivities based on an antiperiplanar conformation.